To cross a stream, highway or valley (where by the hold off due to a flight of locks at both aspect could be unacceptable) the valley may be spanned by a navigable aqueduct – a famous case in point in Wales could be the Pontcysyllte Aqueduct (now a UNESCO https://sergiobhmtw.blogunok.com/37264709/helping-the-others-realize-the-advantages-of-urso-fofo